The present invention pertains generally to thermoelectric devices and, more particularly, to a self-sufficient, low power thermoelectric generator having a compact size and a relatively high voltage output which is specifically adapted to be compatible with microelectronic devices.
The increasing trend toward miniaturization of microelectronic devices necessitates the development of miniaturized power supplies. Batteries and solar cells are traditional power sources for such microelectronic devices. However, the power that is supplied by batteries dissipates over time requiring that the batteries be periodically replaced. Solar cells, although having an effectively unlimited useful life, may only provide a transient source of power as the sun or other light sources may not always be available.
Thermoelectric generators are self-sufficient energy sources that convert thermal energy into electrical energy under established physics principles. The Seebeck effect is a transport phenomenon underlying the generation of power from thermal energy utilizing solid state electrical components with no moving parts. The Seebeck effect utilizes a pair of dissimilar metals (n-type and p-type), called thermocouples, which are joined at one end. N-type and p-type respectively stand for the negative and positive types of charge carriers within the material. If the joined end of the thermocouple is heated while the unjoined end is kept cold, an electromotive force (emf) or voltage potential is generated across the unjoined end. Based on free electron theory of metals, the forces acting on the electrons at the junction of the two dissimilar metals tend to pull the electrons from the metal having a higher electron density toward the metal having a lower electron density. The metal that gains electrons acquires negative electrical potential while the metal that loses electrons acquires positive potential.
The temperature gradient across the thermocouple may be artificially applied or it may be natural, occurring as “waste heat” such as the heat that is constantly rejected by the human body. In a wristwatch, one side is exposed to air at ambient temperature while the opposite side is exposed to the higher temperature of the wearer's skin. Thus, a small temperature gradient is present across the thickness of the wristwatch. A thermoelectric generator may be incorporated into the wristwatch to take advantage of the waste heat and generate a supply of power sufficient to operate the wristwatch as a self contained unit. Advantageously, many microelectronic devices that are similar in size to a typical wristwatch require only a small amount of power and are therefore compatible for powering by thermoelectric generators.
The operating parameters of a thermoelectric generator may be mathematically characterized in several ways. For example, the voltage measured across unjoined ends of a thermocouple is directly proportional to the temperature difference across the two ends. When n-type thermoelectric legs and p-type thermoelectric legs that make up a thermocouple are electrically connected in series but thermally connected in parallel with a temperature differential T1 and T2 maintained thereacross, the open circuit voltage V under the Seebeck effect may be mathematically expressed by the following formula:V=S(T1−T2)where S is the Seebeck coefficient expressed in microvolts per degree (μV/K).
The efficiency of thermoelectric generators may be characterized by a thermoelectric figure of merit (Z), traditionally defined by the following formula:Z=S2σ/κwhere σ and κ are the electrical conductivity and thermal conductivity, respectively. The figure of merit Z, expressed in reciprocal K, represents the thermal and electrical properties of a thermoelectric material that may be utilized in a thermoelectric generator. One of the keys to improve the efficiency of thermoelectric generators lies in the development of highly effective thermoelectric films having low electrical resistance, high Seebeck coefficient and low thermal conductivity.
Another key in improving thermoelectric generators lies in increasing the integration density of the thermocouples. Often with waste heat sources, only a small temperature difference exists between the heat source and the heat sink. Because of this small temperature difference, a large number of thermocouples must be connected in series in order to generate a sufficient thermoelectric voltage. Consequently, the thermocouples must have extreme aspect ratios of length to width of the cross-section.
The prior art includes a number of devices that attempt to improve the efficiency and operating characteristics of thermoelectric generators. One prior art device includes a heat-conducting substrate disposed in thermal contact with a high-temperature region opposite a low-temperature region. Heat flows from the high-temperature region into the heat-conducting substrate and into a number of alternating n-type and p-type thermoelectric legs cut from crystal material. The n-type and p-type thermoelectric legs are electrically connected in series and thermally connected in parallel. The n-type and p-type thermoelectric legs are formed on the substrate in a two-dimensional checkerboard pattern. Because total voltage is the sum of the individual voltages across each n-type and p-type pair, and because each thermocouple of n-type and p-type thermoelectric legs may produce only a few millivolts for a given temperature differential, a very large area is required in order to encompass the checkerboard pattern of alternating n-type and p-type thermoelectric legs. Such a large area requirement prevents the miniaturizing of the thermoelectric generator.
Another prior art device provides a thermoelectric module having a gapless insulating eggcrate for providing insulated spaces for a number of n-type and p-type thermoelectric legs. The absence of gaps eliminates the possibility of interwall electrical shorts between the thermoelectric legs. The thermoelectric legs are electrically connected in series and thermally connected in parallel between hot and cold sides of the module. Electrical connections are comprised of a layer of aluminum over a layer of molybdenum. The surfaces are ground down to expose the eggcrate walls except in the areas where the thermoelectric legs are interconnected. Although the module of the reference overcomes the problems of electrical shorts between adjacent thermoelectric legs, the device of the reference requires numerous manufacturing steps and is therefore costly.
Other prior art devices attempting to miniaturize thermoelectric generators have increased the integration density of thermocouples by miniaturizing the individual monolithic structures of the thermocouples. Although such devices succeeded in reducing the cross section of these bulk material bismuth telluride thermocouples to a sufficiently small size, the extreme difficulty in handling and fabricating these bismuth telluride-type thermocouples from bulk material translates into extremely high production costs leading to a very high cost of the final product.
In view of the above-described shortcomings of conventional thermoelectric generators, there exists a need in the art for a thermoelectric generator that is compatible with the requirements of microelectronic devices. More specifically, there exists a need for a thermoelectric generator for producing low power that is of compact size, and that is specifically adapted for producing a high output voltage while being mass-producible at a relatively low cost.